Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
In an electrochemical cell or battery, an electrolyte element is interposed between the cathode and the anode to prevent the flow of electrons from the anode to the cathode, as would occur in a short circuit. This electrolyte element must be electronically nonconductive to prevent the short circuiting, but must permit the transport of ions between the anode and the cathode during cell discharge, and in the case of a rechargeable cell, also during recharge.
Typically, the electrolyte element contains a porous material, referred to as a separator since it separates or insulates the anode and the cathode from each other, and an aqueous or non-aqueous electrolyte in the pores of the separator. The aqueous or non-aqueous electrolyte typically comprises ionic electrolyte salts and water or electrolyte solvents, and optionally, other materials or additives such as, for example, ionically conductive polymers. A variety of materials have been used for the porous layer or separator of the electrolyte element in electrochemical cells. These porous separator materials include polyolefins such as polyethylenes and polypropylenes, glass fiber and paper filter papers, and ceramic materials. Usually these separator materials are supplied as porous free standing films which are interleaved with the anodes and the cathodes in the fabrication of electrochemical cells. Alternatively, the porous layer can be applied directly to one of the electrodes, for example, as described in U.S. Pat. No. 5,194,341 to Bagley et al. and U.S. Pat. No. 5,882,721 to Delnick; and in Eur. Pat. Application Nos. 848,435 to Yamashita et al.; 814,520 and 875,950, both to Delnick; and 892,449 to Bogner.
U.S. patent application Ser. No. 08/995,089 titled “Separators for Electrochemical Cells,” filed Dec. 19, 1997, to Carlson et al. of the common assignee, describes separators for use in electrochemical cells which comprise a microporous layer of a dried sol, such as a dried pseudo-boehmite sol, and electrolyte elements and cells comprising such separators. These dried sol or xerogel separators and methods of preparing such separators are described for both free standing separators and as a separator layer coated directly onto an electrode.
When a separator layer is coated directly onto an electrode, such as onto the cathode, the porous separator coating may require a relatively smooth, uniform surface on the cathode and also may require a mechanically strong and flexible cathode layer. For example, for a microporous pseudo-boehmite layer having a xerogel structure, smooth, strong, and flexible cathode layer properties may be required to prevent coating non-uniformities, excessive stresses, and possible cracking of the xerogel layer during drying of the sol coating on the cathode surface and also during fabrication and use of electrochemical cells containing the xerogel-based separator. Cracking of the coated separator layer may lead to short circuiting of the cell.
It would be advantageous to be able to prepare electrochemical cells having separators with ultrafine pores and with reduced thicknesses of less than 15 microns that are coated in contact to one or more other layers of the electrochemical cell by a process of coating without cracking or other non-uniformities in the separator that may lead to short circuiting.